1. Field of the Invention
The invention relates to a method for extracting motion errors of a platform carrying a coherent imaging radar system from the raw radar data and device for executing the method.
2. The Prior Art
Coherent imaging radar systems are installed on a platform, such as an aircraft, missile or helicopter or the like. Because of the conditions prevailing in connection with such platforms and because of the effects of their surroundings, such as turbulence, a platform normally cannot maintain a preset flight path. The platform deviates from the desired, set flight path, which is called a motion error. Motion errors of a magnitude of the wave length of the radar transmission signal cause distortions in coherent imaging radar systems, because of which the quality of an image is greatly reduced. The quality of an image is judged by its resolution, the contrast and the geometric distortions.
To attain high resolution, high contrast and low geometric distortions in an image, the raw data received must be corrected prior to processing or generation of the image. Such corrections can be performed in real time or off-line. Real time corrections are performed during reception of backscatter signals with the aid of digital or analog devices. After the raw data have been stored, off-line corrections are made on the ground with the aid of computer programs.
Processing or generation of an image can only be performed after a correction, which is called motion compensation. A correlation between the raw data and the expected theoretical phase history is performed for generating an image, and a high resolution two-dimensional image can only be obtained after such a correlation.
All real time motion compensation systems depend on on-board inertial navigation systems (INS) or other navigation systems, such as GPS (Global Positioning System). With some real time motion compensation systems, an additional simple Doppler analysis of the raw radar data is performed in order to estimate the angle of drift of the platform based on wind effects or because of the scanning geometry. This has been described by way of example in a publication in connection with a CCRS symposium, held in Canada in 1988, in the form of a special issue 88 CH 2572-6/88/000-015 of IEEE.
Either a processing program similar to that used with the real time systems is used with all known off-line motion compensation systems and/or an autofocus method is additionally used during generation of the image.
However, all known motion compensation systems have several disadvantages. Because of the high demands made on the motion data in respect to accuracy, band width and temporal stability, it is necessary to use inertial navigation systems which, for example, have bene specially manufactured, in connection with imaging radar systems where motion compensation is performed. However, this results in very high costs for procurement, installation and maintenance. When using motion compensation systems operating with GPS receivers, support by a ground station is absolutely necessary. However, because of this not only are the operating costs very high, but considerable restrictions in the choice of the field of operations must also be expected. But without the support of a ground station, the GPS motion data absolutely required for motion compensation are not sufficiently exact.
Although autofocus methods are not dependent on inertial navigation systems, these methods cannot be performed in real time because of the great computing demands. Furthermore, autofocus methods do not have a great band width, nor do they have high accuracy, so that motion errors caused in particular by wind gusts cannot be corrected. For this reason, autofocus methods are normally only used to estimate the forward motion of a platform.